Mineral fillers are used on a large scale in paper manufacturing. Their function consists primarily of increasing the opacity of the paper and level of brightness. Relatively inexpensive mineral fillers include china clay, natural calcium carbonate such as ground calcium carbonate, precipitated calcium carbonate, talcum and calcium sulphates. In addition to optical properties, mineral fillers also influence weight, volume, porosity, mechanical properties, particularly bursting strength, the smoothness of the surface, and printing characteristics.
U.S. Pat. No. 5,292,365 discloses a single product which can be used equally as paper filler and as a coating pigment having    a) a rhombohedral or round particle shape    b) a steepness factor (Particle diameter in micrometers at 50% of mass/Particle diameter in micrometers at 20% of mass (d50/d20) of between 1.1 and 1.4    c) a ratio R (volume mass % of particles <1 micrometers/volume mass % of particles <0.2 micrometers) of 8-19 and    d) an average particle diameter of between 0.4 and 1.5 micrometers. The average particle diameter of the invented products is the particle diameter in microns, derived from the X-axis at a value on the Y-axis of 50% mass of the particles.
The top cut is between 4 and 7 micrometers. The term “top cut” refers to the size (in microns) of the coarsest particles of the product. For example, a top cut of 10 microns means that 100% of the particles are smaller than 10 microns. The inventors of the present invention will define the top cut by d98 due to the fact that the 100% line is varying at least ±0.5% far up (error bar).
WO2009009553 discloses precipitated calcium carbonate compositions for coating exhibiting improved opacity, sheet gloss, print gloss, and brightness. The precipitated calcium carbonate compositions are characterized by having a crystalline aragonite content of greater than or equal to about 30% by weight relative to the total weight of the composition. Furthermore, less than or equal to about 10% by weight of particles are having a particle size less than or equal to about 0.25 micrometers. Less than or equal to about 4% by weight of particles have a particle size greater than or equal to about 2.0 micrometers, and a particle size distribution steepness factor ((d30/d70)*100) of greater than or equal to about 50.
US patent application 2006292305 discloses a composition with a first pigment component comprising particulate ground calcium carbonate (GCC) having a particle size distribution (psd) steepness factor ((d30/d70)*100) ranging from about 30 to about 45; and a second pigment component comprising particulate precipitated calcium carbonate (PCC) having a psd steepness factor ranging from about 55 to about 75 and a d50 not greater than 0.5.
Canadian patent 1150908 discloses a calcium carbonate composition with a ratio R (% of particles <1 micrometers/% of particles <0.2 micrometers) greater than 3.5.
EP 1 452 489 A1 discloses a material for imparting thixotropy, the material comprising surface-treated calcium carbonate. The surface-treated calcium carbonate has been prepared by treating a calcium carbonate with fatty acids, resin acids, such as abietic acid, dehydroabietic acid and dihydroabiteic acid; silane coupling agents such as vinylsilane, aminosilane and mercaptosilane; resins such as polyethylene, polypropylene and urethane resins; and polymeric dispersants.